The present invention relates to mass filters, including quadrupole mass filters and more particularly to a glass quadrupole electrode assembly for a mass spectrometer.
Mass filters are tools for analyzing the chemical composition of matter, for example by using electric fields to separate ionized particles by their mass-to-charge ratios. High filtering resolution has been achieved using quadrupole mass filters that include four parallel elongated electrodes. The ideal cross section for the elongated electrodes approximates four hyperbolic arcs extending in their respective quadrants to infinity about a common origin. Generally, only the hyperbolic arcs near the origin are approximated.
FIG. 1 shows a four metal rod implementation of a quadrupole filter. The hyperbolic electrode surfaces are typically formed by grinding the hyperbolic shape from solid metal, e.g., molybdenum or stainless steel rods. The desired arrangement of the four ground rods is then maintained by harnesses of ceramic or other rigid, nonconductive materials.
However, there are several disadvantages to the metal rod implementation of a quadrupole filter, e.g. expense, weight, bulk and vulnerability to misalignment. Grinding identical hyperbolic surfaces on four several inch long molybdenum rods is costly both in terms of time and materials. Further, only the hyperbolic surface is electrically useful. The bulk of the metal rod serves only limited functions such as providing rigidity. Further if the four rods in ceramic harnesses are jolted, misalignment can easily occur. This misalignment may be undetectable by an unaided eye and yet can unpredictably distort the resulting spectra.
One approach which eliminates some of the problems associated with the four metal rod implementation of the quadrupole filter is disclosed in U.S. Pat. No. 4,885,500 to Hansen, et al. U.S. Pat. No. 4,885,500 discloses a glass quadrupole where the electrode assembly structure is provided by an appropriately shaped glass tube which serves as a substrate for the quadrupole. The conductive electrodes are achieved by fusing thin strips of metal to the hyperbolic contours of the inner surface of the glass tube.
The use of a glass quadrupole greatly reduces the size and weight due to the substitution of glass and thin metal strips for the rods in the metal rod implementation. Cost and labor is greatly reduced since glass can be 1) economically obtained and 2) be formed by vacuum formation over a mandrel. The cost and time involved for the formation of a glass quadrupole using a reusable mandrel is reduced compared to the cost and time involved in grinding four metal rods per mass quadrupole filter.
Further, glass tends to be less susceptible than quadrupole metals to small and elastic deformations, so that valid spectra are generally obtainable except when the structural integrity of the glass is breached. Damage to a glass quadrupole is more readily detected visually than damage to a metal quadrupole. Thus, there is less likelihood of a damaged glass quadrupole being operated under the impression that it is providing valid spectra.
Although a glass quadrupole alleviates some of the problems associated with the standard metal rod implementation, there are still problems associated with the glass quadrupole described in U.S. Pat. No. 4,885,500. One problem associated with the mass quad filter described in the aforementioned patent is electrical charge accumulation at the interface between the conductive poles and the insulating dielectric cusps. This accumulated charge creates electric fields that distort the mass selection fields created by the poles.
Ideally the cusp between conductive poles should be infinite to eliminate the effect of charge distortion. Because the cusp distance between the poles are truncated near the active filtering region, the electric field distortion is aggravated. The charge build up is further aggravated at high voltages where the charge cannot dissipate at a rate faster than charge is generated.
A second problem with the glass quadrupole disclosed in U.S. Pat. No. 4,885,500 is field emissions which occur at the interface between the conductive poles and the dielectric. A high voltage at the pole-dielectric interface may result in electron discharge from the conductive material at the pole-dielectric interface into space in the regions surrounding the interface. This electron discharge distorts the axial field and can have secondary ionizations. Electric field distortion is aggravated when the distance between the central axis and the pole-dielectric interface is small.
Other problems associated with the quadrupole described in U.S. Pat. No. 4,885,500 are related to manufacture of the conductive poles. It is difficult to get a smooth edge where the conductive metal strip meets the dielectric and the metal edge is often jagged. The jagged metal edge increases the probability of field emissions at the conductive pole/dielectric interface.
Both the metal rod quadrupole and glass quadrupole disclosed in Hansen et al. are placed inside a vacuum chamber during operation of the mass filter. FIG. 1 shows a isometric view of a mass filter having a four rod electrode assembly. The mass analyzer assembly includes a mass filter assembly 110, an ion source 112 and a detector 114. The quadrupole assembly 110 is enclosed in a chamber 116 to which a vacuum is applied. Electrical connections to a power supply are made through openings 118, 120.
Inherent to a vacuum and the choice of methods and materials used to contain a vacuum, is consideration for diffusion of gases through materials, adsorption and desorption of gases from the surfaces as well as leak paths between surfaces. Additionally, consideration for trapped air, water or other contaminants present between internally mating parts is important as the resulting virtual leaks will adversely affect ion transport and detection. The greater the surface area present in the vacuum and the greater the number of trapped volumes between parts, the greater the degradation in the vacuum and thus in the quality of spectra.
The quadrupole assembly of the mass filter is typically connected to an ion source and an ion detector. Because ion sources and detectors are not standard in size, the interface between the ion source and the quadrupole and the quadrupole and the ion detector must be specifically designed to mate with each quadrupole. A modular design which allows different ion sources and ion detectors to be coupled to the quadrupole assembly is needed.
A mass filter which provides the size, bulk, cost and reliability advantages of a glass quadrupole, yet reduces the effects of charge accumulation and field emissions, improves reproducibility, and which allows for superior vacuum integrity without sacrificing mass filter performance is needed. Concomitantly, it is the objective of the present invention to provide a method of manufacturing such a glass quadrupole.